Many earth boring drill bits and particularly those commonly known as drag bits, are designed currently with cutting surfaces comprising a plurality of polycrystalline diamond faced cutters, each mounted on a tungsten carbide stud. The drill bit includes apertures or holes adapted to receive the studs. A significant effort has been made to distribute the individual cutters about the drill bit to provide the most efficient operation. In particular, a design goal is to maintain uniform wear on the cutters to maximize the service life of the drill bit. To obtain such uniform wear, each individual cutter should cut equal volumes of material.
In the past, designers have spaced cutters at uniform increments along radii extending from the central axis of the drill bit. This design has not been found to be completely effective. An attempt to empirically determine an optimum cutter distribution was made in a technical paper entitled "Optimization of Radial Distribution of Stratapax Cutters and Rock Drilling Bits" by J. D. Barr. This paper was presented to the Energy-sources Technology Conferences, at New Orleans in February, 1980. In this paper, a power law model was assumed to relate cutter wear rate to cutter velocity and area of cut. In the paper, the distribution of cutters was made according to the following formula: EQU 1/S=K.sub.3 R.sup.E
where E=b/c
In this formula, S is the radial spacing between cutters, K.sub.3 is an empirical constant, R is the distance of the cutting edge from the central axis of the bit, E is a spacing exponent, b is a velocity exponent and c is an area exponent. An upper limit for the value of S is selected at about 1/3 or 1/4 of the cutter diameter. This ensures adequate redundancy in the event of loss of a particular cutter, or even two adjacent cutters. A disadvantage of the design method disclosed in this paper is the requirement for empirical wear measurements from used bits in the particular material to be drilled. Also, experiments undertaken by the author of the paper and summarized therein resulted in considerable scatter from theoretical wear patterns.
Another approach to the distribution of cutters on drill bits is described in a publication released by Sandia Laboratories. This publication is entitled "Stratapax.TM. Computer Program" and authored by Richard F. Ashmore et al. The publication is identified by a number SAND77-1994 and was printed in April, 1978.
This publication records a complex computer program which calculates the volume of material cut by each cutter on a drill bit. A number of variables can be input into the computer program. These variables include the location of the cutter in radial distance from the central axis of the bit, the angular location of the individual cutters from an arbitrary base line in a plane perpendicular to the central axis of the bit and the position of the individual cutters along the length of the bit. The computer program can optimize the positioning of a number of cutters by trial and error iteration to achieve equal volume cut for each cutter.
However, the computer program has several disadvantages. An operator must first select a pattern of cutter distribution to initialize the computer program. The initial selection of cutter position affects the usefulness of the iteration technique. In addition, sophisticated and expensive computer facilities must be available to the designer.
A need exists for a simplified, yet effective design for placement of the cutters on the drill bit. The design should avoid the complications of empirical data which require expensive and time consuming test programs. In addition, the design should avoid complex and expensive calculations which require specialized and expensive computing equipment.